The work proposed would apply a general method of selection in the histidine operon based upon expression in Salmonella typhimurium/Escherichia coli partial diploids to develop and characterize unique his mutants. These mutants include deletions of part of all of the first structural gene and in some cases extend from the distal side into the histidine operator- promoter region; deletions of the hisG-hisD boundary; and secondary promoter mutations internal to the his opron. Studies would include fine genetic structure, enzyme assays, and manipulation of his alleles into defective lysogens suitable for DNA and mRNA analysis in the characterization of genetic transcription signals.